We have 4 kids. Before we had our 3rd, we needed to buy a new vehicle solely because we couldn't fit 3 car seats into the back of our old car. And when traveling with kids, carrying 4 gigantic car seats plus your other luggage is not exactly as easy as you might think! It essentially rules out solo parent travel with all 4 kids. Transferring car seats between two cars, or installing car seats in a taxi, is a serious pain.

Furthermore, the evidence that car seats actually benefit safety is significantly less robust than you might think. The "mountains of evidence" that do exist for things like 70% reductions in fatalities, bizarrely enough, generally compare the rate of fatalities for car seats vs completely unrestrained kids. When you compare the rate of fatalities in car seats to kids wearing adult seat belts, the bulk of the evidence suggests essentially no difference. Fatalities happen when the forces involved are catastrophic and sadly a car seat doesn't help much for kids over 2.

Even a back of the envelope comparison makes this extremely plausible: car crash fatalities for kids 9-12 have declined by 72% from 1978-2017. If car seats and car seat laws save significant numbers of lives, you'd expect that the fatality rate for kids 0-8, who are generally in car seats, to have decreased much more. But it hasn't, it declined by 73% over the same period.

Now, car seats and boosters do seem to moderately reduce non-fatal injuries - huge spread of estimates here, most clustering around 10-25%. It's reasonable for most people to use car seats or boosters most of the time based on this alone, IMO, especially for young kids. But do they justify a mandate? IMO: no. Absolutely not.

Worth mentioning that mandates probably do succeed in one thing: they reduce the number of children born at all by at least 57x more than they prevent child fatalities. Roughly 8,000 kids per year, 145,000 kids since 1980. That's with the (unlikely, as discussed above) assumption that car seats do in fact save significant numbers of lives. But it's also entirely possible that they've prevented hundreds of thousands of kids from being born, somewhat reduced the nonfatal injury rate, and saved essentially no lives.

Citations below:

Fatality reduction with car seats or boosters:

- https://pricetheory.uchicago.edu/levitt/Papers/seatbelts.pdf (found that seat belts as effective as car seats for children 2-6)

- https://onlinelibrary.wiley.com/doi/10.1002/jae.2449 (independent replication of above with different data set)

- https://pubmed.ncbi.nlm.nih.gov/19959729/ (no statistically significant difference between booster seats and seat belt alone for fatalities)

- https://pubmed.ncbi.nlm.nih.gov/16754824/ (the main counter-estimate to the above, with the 28% fatality reduction)

Non-fatal injury reductions:

- https://ideas.repec.org/a/bla/ecinqu/v48y2010i3p521-536.html (no difference in serious injuries, ~25% reduction in least serious injury category)

- https://crashstats.nhtsa.dot.gov/Api/Public/ViewPublication/... (14% reduction in likelihood of injury for boosters)

- https://pubmed.ncbi.nlm.nih.gov/19841126/ (45% estimate)

- https://pubmed.ncbi.nlm.nih.gov/12783914/ (59% estimate)

Reduction in birth rate from car seat mandates:

- https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3665046 (car seat mandates "led to a permanent reduction of approximately 8,000 births in the same year, and 145,000 fewer births since 1980, with 90% of this decline being since 2000")

Note that both the 45% and 59% estimate for injury reduction and the 28% estimate for fatality reduction all come from one research group using a proprietary data set. Everything that's independently reproducible points towards small or zero effect on fatalities and modest effects on injuries.